1. Field of the Invention
The present invention relates to synthetic resin emulsion powder. More precisely, the invention relates to synthetic resin emulsion powder obtained by spray-drying a composition that comprises an emulsion (A) where the dispersoid is a polymer having one or more unsaturated monomer units selected from ethylenic unsaturated monomers and dienic monomers, and a polyvinyl alcohol (B) having an ethylene unit content of from 1 to 12 mol %.
2. Description of the Related Art
Synthetic resin emulsion powder is produced by spray-drying a synthetic resin emulsion, and is superior to the starting emulsion thereof in point of the processability and the transportability as it is powdery. Before use, water may be added to the powder and stirred, whereby the powder may readily redisperse in water. Accordingly, the powder is much used in various applications for additives to cement and mortar, adhesives, binders for coating compositions, etc. In particular, since the powder can be premixed with mortar to realize various modifications of commercial products, and it is especially favorable for additives to mortar and is therefor widely used in the art. However, when conventional synthetic resin emulsions are directly spray-dried, then the dispersoid particles therein may readily fuse together and could not redisperse in water. At present, therefore, a large amount of polyvinyl alcohol must be added later to the emulsions, or a large amount of inorganic powder such as silicic anhydride that serves as an antiblocking agent must be in the emulsions to solve the problem. For the polyvinyl alcohol that is added later to the emulsions, partially saponified (hydrolyzed) PVA has heretofore been widely used (see JP-A 11-263849, claim 1 and paragraphs [0011] and [0012]) since it must be powdered and must be re-emulsified before use. However, as will be obvious from Comparative Examples 7 and 8 given hereinunder, the redispersibility of the PVA powder is not always good, and, in addition, another problem with the PVA powder is that the water resistance of the emulsion obtained through redispersion of the emulsion powder is inferior to that of the original emulsion.
Also known is a powder for that purpose, that is obtained by spray-draying an emulsion with a mercapto-terminated polyvinyl alcohol serving as a dispersant (see JP-A 9-151221, claim 1 and paragraphs [0011] and [0020]). Further known is a powder that is obtained by spray-drying an emulsion with a 1,2-glycol bond-rich PVA which is prepared through high-temperature polymerization and serves as a dispersant (see JP-A 2001-342260, claim 1). The redispersibility of these synthetic resin emulsion powders is good, as is apparent from Comparative Examples 26 to 27 given hereinunder, but is not still satisfactory. For example, when the powders are used as additives to cement mortar, the mechanical strength of the resulting cement mortar is not always satisfactory.